This invention relates to an expandable downhole tool for incorporation in a drillstring, such as an under-reamer or a stabiliser.
Under-reamers are normally used to enlarge a borehole below a restriction, to result in a borehole that is larger than the restriction. Stabilisers are used to control the direction of a drill bit during the drilling process.
The present invention relates to an expandable tool that may function as an under-reamer, or alternatively may function as a stabiliser in an undereamed portion of a borehole.
It will be well known to those of ordinary skill in the art, in the drilling of oil and gas wells, to employ under-reamers and stabilisers, and detailed description of their normal function is not considered necessary.
U.S. Pat. No. 6,732,817 describes a downhole tool that functions as an underreamer, or alternatively, as a stabilizer in an underreamed borehole. The tool comprises one or more moveable arms disposed within recesses in the tool. The tool alternates between collapsed and expanded positions in response to differential fluid pressure between the flowbore and the wellbore annulus. The recesses comprise angled channels that slidably engage extensions formed on the arms such that when the tool expands, the arms are translated axially upwardly, while simultaneously being extended radially outwardly from the body.
The apparatus described in U.S. Pat. No. 6,732,817 suffers from the drawback that the recesses must be formed with integral angled channels, which increases the complexity and cost of the tool. Also, if the angled channels in the recesses become damaged or subject to wear and tear, they are relatively costly and difficult to replace. The apparatus of U.S. Pat. No. 6,732,817 also suffers from the drawback that the cutter blocks do not lock in the outward position such that the blocks can float up and down on inclined plains causing vibration and wear and tear. This can also result in the tool not cutting a well bore to the desired size.